The invention more particularly relates to those devices that include circuits of stimulation and detection at the same time on the atrium and on the ventricle, and that can operate according to at least two operating modes, DDD or AAI. These devices can be equipped with a mode called “AAISafeR,” which ensures an automatic commutation from DDD mode to AAI mode and vice versa.
The basic operating mode of a DDD/AAI pacemaker is an AAI mode, with an atrial stimulation and a monitoring (detection) of the ventricular activity. This mode is maintained as long as atrio-ventricular conduction is normal, i.e., as long as each atrial event (atrial detection, corresponding to a spontaneous activity, or atrial stimulation) is followed by an associated ventricular detection.
In certain circumstances, however, atrio-ventricular blocks (AVB) can appear, involving a temporary disorder of depolarization of the ventricle. When this happens, the pacemaker switches automatically to DDD mode, with parameters optimized for the temporary AVB situation. After disappearance of the AVB, and thus re-establishment of atrio-ventricular conduction, when a certain number of conditions are filled the pacemaker turns over automatically to AAI mode. This commutation between DDD and AAI modes is described in EP-A-0 488 904 and its counterpart, U.S. Pat. No. 5,318,594 (incorporated herein by reference), and EP-A-1 346 750, its counterpart U.S. Patent App. No. US2004010292, all of which are assigned commonly herewith to ELA Médical, as well as in U.S. Patent Application Nos. 20020082646 and 20030078627, which are assigned to Medtronic, Inc.
The starting point of the present invention lies in observations carried out at the time of a clinical follow-up of patients equipped with DDD/AAI devices with automatic mode commutation (also called automatic mode switching). It appeared that these apparatuses have an insufficient specificity according to the type of AVB, so that in certain cases inappropriate commutations were occurring towards DDD mode, leading to unjustified alternations of commutations AAI towards DDD then DDD towards AAI, or to final commutations, in DDD mode, which were useless. More precisely, one classically distinguishes three degrees of AVB, corresponding to an increasing gravity of the disorder of conduction:                a) The AVB of the first degree (AVB1) corresponds to a conduction present, but delayed; the commutation towards DDD mode is started when the number of atrial events followed by a ventricular detection occurring after a delay longer than, e.g., 350 ms (for a spontaneous atrial event) or 450 ms (for a stimulated atrial event) exceeds a given number, e.g., in the event of detection of six consecutive cardiac cycles fulfilling this criterion.        b) The AVB of the second degree (AVB2) is characterized by an incomplete conduction, the progressive lengthening of the interval PR (or AR) being such that a part of the P waves are no longer conducted. Commutation to DDD mode on an AVB of the second degree is typically started when the number of atrial events not followed by a ventricular detection exceeds a certain number over the duration of a window of monitoring extending over a predetermined number of atrial events. In other words, commutation to, e.g., DDD is started when the device detects three nonconsecutive blocked P waves among the last twelve cardiac cycles.        c) The complete AVB, or AVB of the third degree (AVB3), which is the most serious, appears by atrial waves (stimulated or spontaneous) completely blocked, i.e., which are not followed any more by ventricular depolarization; the device must then quickly operate the commutation to DDD mode, this commutation intervening typically when it detects a succession of two blocked atrial waves (detected or stimulated), or if it times out by more than three seconds without ventricular detection.        
It also is necessary to take into account:                d) The ventricular pause, which may find its origin in a disorder of atrio-ventricular conduction. There is ventricular pause when the interval separating two ventricular events exceeds a given time, e.g., exceeds three seconds.        
After a commutation to DDD mode on an AVB, to return to AAI mode the device waits until a certain number of return criteria are fulfilled, e.g., it returns to AAI mode after one hundred cycles with ventricular stimulation (in order to be able to test, while returning to AAI mode, if a spontaneous atrio-ventricular conduction is restored) or twelve consecutive cycles with detection of a spontaneous depolarization of the ventricle. In addition, it may be desirable to envisage a limitation of the number of successive commutations from AAI towards DDD over a given period. If, e.g., the device started more than fifteen commutations over a 24 hour period or if, over a three consecutive day period, the device started more than five commutations over 24 hours, then it switches definitively to DDD mode and functions on the basis of a programmed parameter of stimulation (in particular the atrio-ventricular delay), this configuration being preferably maintained until there is a new examination of the patient and possible reprogramming by the physician. The above-identified rules of commutation may, however, in certain cases be inappropriate.
The first case is that of “troubles of the atrial rate” (TdRA), a generic term which covers various atrial arrhythmias (nonphysiological episodes of acceleration of the rate) such as tachycardia, fibrillation, flutter, etc., which are all characterized by detection of a fast atrial rate. In such a case of suspected or proven TdRA, the above criteria of detection of AVB1, AVB2, or AVB3 are no longer appropriate because the same cardiac cycle generally lets several atrial events appear, and the multiple detections distort the analysis of atrio-ventricular conduction. On the other hand, the criterion of detection of ventricular pause remains relevant.
In addition, the AVB very often presents an intermittent character, particularly the AVB1 and AVB2, which can, in certain situations, present a simply paroxystic character, i.e., they can occur commonly, in particular during phases of effort or during sleep, and disappear spontaneously at the end of the effort or upon awakening. In these situations of paroxystic AVB, with the known devices functioning in the manner indicated, one can note a certain number of disadvantages:                A final commutation in DDD mode on detection of paroxystic AVB at night is not a priori relevant because the patient can also present a satisfactory atrio-ventricular conduction during the day or at rest.        During a same phase of effort, it would be desirable to avoid successions of commutations AAI towards DDD then DDD towards AAI. Indeed, if there were, e.g., a commutation AAI towards DDD during the effort, one can reasonably think that the AVB that started this commutation will persist until the end of the effort, and that attempts at re-commutation in AAI are not suitable as long as the effort persists, because they are likely to involve symptomatic pauses at the time of this exercise.        On the other hand, when the effort is finished, it is desirable to be able to return as quickly as possible to an AAI operation because, if the AVB were only paroxystic, the A-V conduction should normally reappear.        It can be excessive to generate a final commutation after successive commutations very close from/to each other (in the event of an episode of conduction disorder), because this episode can be unique over a given period, e.g., an episode of block during one hour duration over one day.        In addition, little clinical information being available on the “paroxystic” character of the AVB (their duration, in particular, is little known), it may be excessive to operate a final commutation caused by a succession of commutations happening during the same episode of conduction. It is necessary, however, to discriminate the chronic AVB, in order to be sure that the apparatus will end up with a commutating to the DDD mode if the conduction disorder episode lasts several days.        Finally, during the effort the heartbeat rate accelerates, and from a hemodynamic view it is not desirable to let the interval PR (or AR) lengthen up to 350 ms (or 450 ms) before starting commutation towards DDD mode. These values of 350 or 450 ms are programmable, but they are fixed whatever the frequency and do not account for the detection of an AVB1, for the particular state of the patient, in phase of effort or not.        